A tap changer is known from DE 20 21 575 that comprises, in total, four vacuum-switching tubes per phase. Provided in each of the two load branches are a respective vacuum-switching tube as a main contact and a respective further vacuum-switching tube, connected in series with a switching resistor, as a contact resistor.
When uninterrupted load changeover from the previous winding tap n to a new, preselected winding tap n+1 takes place initially the main contact of the side switching off is opened and thereupon the contact resistor of the side taking over closes so that a compensating current limited by the switch-over resistors flows between the two taps n and n+1.
After the previously closed contact resistor of the side switching off has opened, the main contact of the side taking over then closes so that the entire load current is conducted from the new winding tap n+1 to the load diverter; the changeover is concluded.
However, in various cases of use such known tap changers with vacuum-switching tubes for regulation of power transformers a high surge voltage strength, up to 100 kV and significantly above that, is required. Such undesired surge voltages, the level of which is substantially dependent on the construction of the tapped transformer and the winding parts between the individual tap stages, are on the one hand lightning surge voltages that result from lightning strikes in the mains. On the other hand, switching surge voltages caused by unpredictable switching surges in the mains to be regulated can also occur.
In the case of insufficient surge voltage resistor of the tap changer a temporary tap short circuit or an undesired break-through at the ceramic member or the attenuating screen of vacuum-switching tubes in the load branch not conducting the load current can happen that not only can cause long-term damage thereof, but in general is undesired.
This leads in many cases in the design of the switching paths and thus particularly also of the vacuum-switching tubes to over-dimensioning so that these reliably withstand the described voltage loading. Not only the small constructional space currently available with modern apparatus, but also the economics as well as serviceability of such vacuum-switching tubes cause such over-dimensioning to appear disadvantageous.
A tap changer is proposed in DE 10 2010 024 255 (not prior-published) in which a first winding tap of a first main current branch is connected with a load diverter by way of a series connection consisting of a first mechanical changeover switch and a first switching means, i.e. a vacuum-switching tube or alternatively a semiconductor component. In a symmetrical form of construction analogous thereto a second winding tap of the second main current branch is similarly connected with the load diverter by way of a series connection consisting of a second mechanical changeover switch and a second switching means, i.e. a second vacuum-switching tube or alternatively a second semiconductor component. Moreover, a first auxiliary current branch with an included resistor is branched off between the first winding tap and the first mechanical changeover switch, by means of which an electrical connection with the second mechanical changeover switch of the second main current branch is producible, and a second auxiliary current branch with a further included resistor is branched off between the second winding tap and the second mechanical changeover switch, by means of which an electrical connection with the first mechanical changeover switch of the first main current branch is producible.
In other words: a mechanical changeover switch that is connected in series with the respective vacuum-switching tube and that ensures complete electrical separation of the respective unconnected winding tap and thus a high surge voltage resistor, is present in the known tap changer in each main and auxiliary current branch.
All tap changers known from the prior art require several vacuum-switching tubes and additional mechanical switching elements per phase that due to the large amount of space required by the individual switching means and the accompanying constructional mechanical outlay is disadvantageous and, above all, costly. Not least this is also because for a changeover process a plurality of requisite individual components is needed in the tap changer for switching sequence realization that then have to interengage within a few tenths of a second in a changeover process precisely defined in terms of time.